US 7438781 B2
A system and method for vacuum bag assembly fabrication via the vacuum bag method which reduces the amount of trapped air. The inventive system includes a chamber adapted to contain a vacuum bag and first and second evacuating mechanisms. The first evacuating mechanism serves to evacuate the bag and vent to atmosphere. The second evacuating mechanism serves to evacuate and vent the chamber to atmosphere. Thus, generally, the inventive method includes the steps of placing a vacuum bag lay up in an air tight chamber; and evacuating the vacuum bag lay up and the air tight chamber at the same time. In practice, the assembly would consist of at least two components and a heat curable composition. The components can be parts to be bonded together or layers of cloth to be made into a laminate. In this case, the heat curable composition would be a catalyzed adhesive or resin.
1. A method of fabricating an assembly of at least two components and a heat curable composition comprising the steps of:
a) providing a vacuum bag lay up; said lay up including said components and said curable composition;
b) placing said vacuum bag lay up in an air tight chamber;
c) evacuating said vacuum bag lay up and said air tight chamber at the same time;
d) maintaining vacuum on said vacuum bag lay up and said air tight chamber for a first period;
e) venting said chamber to atmosphere;
f) maintaining vacuum on said vacuum bag lay up for a second period;
g) moving said vacuum bag lay up to an oven;
h) curing said curable composition;
i) venting said vacuum bag lay up to atmosphere; and
j) removing said assembly from said vacuum bag lay up.
2. The invention of
3. The invention of
4. The invention of
5. The invention of
6. The invention of
7. The invention of
a. increasing temperature to cure temperature for said curable composition; and
b. maintaining said cure temperature for a sufficient time to effect complete cure;
whereby adequate but less than optimum interlaminar bond strength is developed so that, if necessary, said components may be easily reworked.
8. The invention of
the curing step includes the further steps of
increasing temperature at a rate of about 7-12° C. per minute from room temperature to about 100-125° C.; and
maintaining the cure temperature for about 10-20 minutes wherein a resulting lap shear strength of the assembly enables components thereof to be easily disassembled with no damage.
9. The invention of
the first period is about 20-30 minutes.
10. The invention of
the second period is about 8-12 minutes.
11. A method of fabricating an assembly of at least two components and a heat curable composition comprising the steps of:
forming a vacuum bag lay up with the components and the curable composition;
placing the vacuum bag lay up in an air tight chamber;
evacuating the vacuum bag lay up;
evacuating the air tight chamber after evacuating the vacuum bag lay up;
maintaining vacuum on the vacuum bag lay up and the air tight chamber for about 20-30 minutes;
venting the chamber to atmosphere;
maintaining vacuum on the vacuum bag lay up for about 8-12 minutes;
moving the vacuum bag lay up to an oven;
curing the curable composition;
venting the vacuum bag lay up to atmosphere; and
removing the assembly from the vacuum bag lay up.
This is a Divisional Application of U.S. patent application Ser. No. 10/106,313, filed 03/26/2002 by David F. Ciambrone et al and entitled SYSTEM AND METHOD FOR VACUUM BAG FABRICATION, now abandoned.
This invention was made with Government support under Contract No. F33657-91-C-0006 awarded by the Department of the Air Force. The U.S. Government has certain rights in this invention.
1. Field of Invention
This invention relates to the field of adhesive bonding and laminating. Specifically, the present invention relates to systems and methods for bonding and laminating using atmospheric pressure, commonly known as vacuum bagging.
2. Description of the Related Art
Vacuum bagging is one of a variety of methods used for the bonding and laminating of manufactured parts. Generally speaking, the part being bonded or laminated is surrounded by an airtight membrane or diaphragm; a vacuum is pulled on that membrane; and the surrounding pressure forces the membrane tightly to the part. Once the part is vacuum bagged it is allowed to cure for a specific period of time depending on its composition. Curing can take place in an oven, autoclave, at room temperature or in a mechanical press.
In adhesive bonding two parts are assembled with a layer of uncured adhesive between them. Typically, a film adhesive is used. The process then applies atmospheric pressure (up to 14.7 psi, depending on altitude). The pressure forces the adhesive layer and the parts into intimate contact thus ensuring good bonds. The vacuum draws out some volatiles and trapped air, resulting in a low void content. Both help to improve resin flow. Vacuum bagging produces strong parts with little air entrapment. The process is adaptable to complex, three-dimensional shapes thus eliminating the need for expensive matched-die molds and presses.
All vacuum bags are constructed in essentially the same way. Before starting a release agent is applied to the tool. This may be a liquid release coating, a wax, or even a solid barrier such as TeflonŽ tape. When applying the release, the edges of the tool must be masked so the bag sealant tape (BST) will stick to the tool.
BST is a putty-like material, which comes in rolls, usually ˝ inch wide, with a release paper on one side. The tape is pressed against the tool, leaving the release paper on until it is time apply the bag. The tape usually goes on after the assembly to be bonded is placed on the tool.
Once the assembly is in place the bag is applied. The first item to go down is a peel ply, which is optional. Peel plies are a tightly woven fabric, often nylon, usually impregnated with some type of release agent. The peel ply may stick to the assembly, but it will pull away without too much difficulty. For adhesive bonding, the peel ply is typically a solid sheet.
After the peel ply comes a layer of release film. This is a thin plastic, which has been treated so it will not bond to the laminate. It is highly stretchable so it can conform to complex geometries.
At least one layer of bleeder cloth goes above the release film. Bleeder is a thick, felt-like cloth. It provides a continuous air path for pulling the vacuum. If the bag wrinkles against the hard assembly, it will trap air. The breather prevents this from happening.
The bag is the last item to be placed. It is a relatively thick plastic layer, available in different amounts of conformability. The bag is usually applied along one edge at a time. One starts at one corner and presses the bag into the BST, removing the release paper from the tape as one moves along the edge. It is important to be careful not to get any wrinkles in the bag or it will leak. Pleats are required for anything but flat or simply curved structures. The bag may be heat sealed instead of using the BST.
Finally, a vacuum port is installed through the bag. The base of the port goes inside the bag. A small cross is cut in the bag for the attachment flange to fit through. If the tool has an area for the port, there must be a breather path from the port to the part. If the port goes on the part itself, several layers of breather are placed under the port to prevent print-through.
While vacuum bagging is a good and inexpensive method of fabricating adhesively bonded assemblies, because of the relatively low pressures employed, a fairly thick bond line results and air can still be trapped in the bond line.
The main aim of adhesive bonding is to produce durable adhesion of one part to another via a layer of adhesive. Ideally, the parts should not be separable from each other for the life of the assembly. However, good adhesion makes rework difficult. If the adhesion is too good, it may not be possible to disassemble the parts even with heating of the adhesive.
Hence, a need remains in the art for 1) a system or method for performing the vacuum bag process which reduces the amount of trapped air and 2) a system or method for providing adequate adhesion while ensuring that the assembly can be easily reworked.
The present invention is a system and method for vacuum bag assembly fabrication via the vacuum bag method which reduces the amount of trapped air. The inventive system includes a chamber adapted to contain a vacuum bag and first and second evacuating mechanisms. The first evacuating mechanism serves to evacuate the bag and vent to atmosphere. The second evacuating mechanism serves to evacuate and vent the chamber to atmosphere.
In practice, the assembly would consist of at least two components and a heat curable composition. The components can be parts to be bonded together or layers of cloth to be made into a laminate. In this case, the heat curable composition would be a catalyzed adhesive or resin.
Thus, generally, the inventive method includes the steps of placing a vacuum bag lay up in an air tight chamber; and evacuating the vacuum bag lay up; and evacuating the air tight chamber. It is important that the vacuum bag lay up and the air tight chamber be evacuated at the same time.
In a more specific embodiment, the inventive method includes the following steps:
Curing may be accomplished by subjecting the curable composition to sufficient heat for a sufficient time to effect a complete cure with optimum interlaminar bond strength. This is the standard method. Adhesive and resin manufacturers always provide the optimum cure schedules for the materials they provide. However, in an improvement on the standard method, which provides for easy reworking of the assembly via producing less than optimum interlaminar bond strength, especially an assembly of bonded components, the temperature is increased at a fast rate to specification cure temperature. The preferred rate is about 8-12° C. per minute. The double vacuum bonding method disclosed herein results in nearly void free bond lines. This enables utilization of adhesive joints with lesser strength adhesives or with thinner bond lines.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
The lay up 10 just described is generic. It can be used for lamination of composite materials or adhesive bonding of several parts. Lamination, as illustrated in
Adhesive bonding, as illustrated in
Bond lines produced during adhesive bonding using the vacuum bag process are typically 98-100% bond free. Those familiar with the art to which this invention pertains will realize that the above description is merely an outline of the prior art and there are many details that are developed when making use of the vacuum bag process to fabricate any particular assembly 22.
Another aspect of this invention is the ability to produce a bond that is amenable to reworking. Normally, heating to a specific temperature for a specific time cures adhesives. For example, the manufacturer's recommendation for curing Arlon ThermalBond NP, a silicone adhesive manufactured by Arlon, Inc., Bear, DE, is 5- 10 minutes at 100-150° C. When cured at this cure schedule, the adhesive typically develops a lap shear strength of 200-300 psi. In the improvement of this invention, the adhesive is raised to a specific temperature at a specific ramp rate and then held at that temperature for a specific time. It has been found that, with Arlon ThermalBond NP, raising the bond line temperature from room temperature to 100-125° C. at a rate of 7- 12° C. per minute and maintaining temperature for 10-20 minutes produces lap shear strengths in the range of 130-185 psi. A programmable oven 78 is required for this aspect of the invention. As can be seen, the lap shear strength developed by use of this invention is less than manufacturer's specification. Subsequent testing of this invention with actual parts showed that the parts could be easily disassembled with no damage. In other words, this method yields a process by which parts are easily reworkable.
It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.